The present invention relates to an X-ray sensor signal processor for processing output signals of X-ray sensors for non-destructive inspection, and an X-ray computed tomography system (hereinafter referred to as “X-ray CT system”) using the X-ray sensor signal processor. Particularly, it relates to an X-ray sensor signal processor in which a dark current generated in each of X-ray sensors can be removed and a value proportional to the average number of photons in X-rays can be obtained even in the case where a small number of incident photons are given, and an X-ray CT (computed tomography) system using the X-ray sensor signal processor.
Industrial X-ray CT systems using X-ray pulses with high energy (of 1 MeV or higher) have been developed in recent years for the purpose of non-destructively inspecting internal defects, or the like, of metallic parts or devices. Greater progress in research and development has been made for the purpose of inspecting a larger object with high resolution.
An X-ray CT system has been described in H. Miyai, et al. “A High Energy X-Ray Computed Tomography Using Silicon Semiconductor Detectors”, 1996 Nuclear Science Symposium Conference Record, Vol. 2, pp. 816-820, Nov. 2-9, 1996, Anaheim, Calif., USA (1997) (hereinafter referred to as “first background art”). A signal processor for processing output signals of X-ray sensors shown in the first background art will be described below with reference to FIG. 9.
In the signal processor 1 shown in FIG. 9, semiconductor sensors (X-ray sensors) 21 to 2n are connected to first-stage circuits 90 to 9n respectively. Because a semiconductor sensor for an X-ray computed tomography is shaped like a strip of paper with a large size (for example, 3×40×0.4 mm) to detect a high-energy X-ray pulse efficiently, there is a possibility that a dark current with a high level of the order of tens of nA may be inevitably generated in the semiconductor sensor. In the first-stage circuit 90, a capacitor 114 is provided to AC-couple the semiconductor sensor 21 so that a voltage amplifier does not amplify a DC voltage caused by the dark current. When X-rays become incident onto the semiconductor sensor 21, an electric current flows through a capacitor 114 because electric charges are generated in the inside of the sensor. A voltage change generated on this occasion is amplified by two stages of voltage amplifiers 92 and 92′, held by a sample/hold amplifier 94 and supplied to a posterior device.
FIG. 10A shows an output of the semiconductor sensor 21 in the first background art, and FIG. 10B shows an output of the sample/hold amplifier 94 in the first background art. An X-ray pulse is output with a pulse width Tw. Hence, when there is no object or when an object penetrated by the X-ray pulse is thin, the output of the X-ray sensor is provided as a rectangular-wave output 101 (solid line) proportional to the dose of the X-ray pulse as shown in FIG. 10A. Hence, the output of the sample/hold amplifier 94 is obtained as an output 103 proportional to the average number of photons per unit time except the first rising portion as shown in FIG. 10B.
On the contrary, when an X-ray pulses pass through a thick object, the number of incident photons is reduced by at least four digits compared with the case where no object is set, because the X-ray pulses are attenuated before the X-ray pulses become incident on the semiconductor sensor 21. The output 102 in FIG. 10A shows an example of the output of the semiconductor sensor 21 in the case where the object is so thick that a small number of incident photons are given. As shown in FIG. 10A, the output of the semiconductor sensor 21 is not kept constant but it has a waveform the output height of which is heightened only when photons are incident on the semiconductor sensor 21. In this case, the output of the sample-and-hole amplifier 94 is not kept constant, either, as represented by the output 104 in FIG. 10B. That is, the value held by the sample/hold amplifier 94 does not always express a voltage proportional to the average number of photons per unit time. As described above, the first background art has a problem that a voltage proportional to the average number of photons per unit time is not obtained when a small number of incident photons are given.
A technique for obtaining a voltage proportional to the average number of photons even in the case where a small number of incident photons are given has been described in JP-A-58-15847 (hereinafter referred to as “second background art”). In the second background art, there has been described a medical X-ray tomography system in which a voltage proportional to the average number of photons is obtained by a root mean square of fluctuation components obtained as a result of removal of a DC component from an output signal of an X-ray sensor by a capacitor. This technique is called “root mean square voltage technique” or “Campbell's technique”.
Assume now the case where the second background art is applied to the first background art. Generally, the pulse width of an X-ray pulse used in the industrial X-ray CT system which is a subject of the first background art is set to be small (about 5 μs) compared with the medical X-ray tomography system which is a subject of the second background art. The pulse with such a small pulse width, however, cannot be removed by a capacitor. Hence, fluctuation components cannot be taken out exclusively by a capacitor even in the case where the second background art is applied to the industrial X-ray CT system which is a subject of the first background art. Because a voltage proportional to the average number of photons cannot be obtained accurately by the root mean square voltage technique if fluctuation components cannot be taken out exclusively, a voltage proportional to the average number of photons cannot be obtained accurately even in the case where the second background art is applied to the first background art as described above.